Threadwatch has a nice catch that this page from Google on AdWords traffic estimates looks different from the cached version recorded by Google's spider. In particular, the HTML title tag of the page humans see says:
Google AdWords Support: Why do traffic estimates for my Ad Group differ from those given by the standalone tool?
Whereas the title tag of the cached version says:
traffic estimator, traffic estimates, traffic tool, estimate traffic Google AdWords Support: Why do traffic estimates for my Ad Group differ from those given by the standalone tool?
What's going on? The first thought is that Google wants this page to rank well for terms like "traffic estimator" or "traffic estimates" and so has put them in the title tag -- but doesn't want that bad looking title to show up to those reading the page, so it's cloaking it.
It could also be that the title previously said this and has since changed. The cached version of the page is dated March 6 as of 5AM GMT. The difference between the cached copy and the current one was spotted on March 7. It is possible, unlikely but possible, that the page was changed within a day and that the Google spider hasn't yet caught up with it.
I'm checking with Google to find out what they have to say and will update as I hear. For more, see these Threadwatch posts: Google Caught Cloaking - Keyword Stuffing Titles and the follow-up Are Google Cloaking and Keyword Stuffing?